How to maintain per-node instance state?
Generally it is not a good idea to store the state of a node in Animation Nodes. The execution of a node should not depend on the executions before. This is also important because nearly all nodes have to work inside of loops - so they can be executed many times with different input in the same node tree execution.

Beside that I agree that it is sometimes useful to store some information (cache data etc.) which cannot be stored in Blenders properties (bpy.props). You cannot use the python dictionary of a node as you noticed. However, it is possible to create a dictionary in the module (file) that contains the node. You can use the identifier of a node (myNode.identifier) as key in the dict. Animation Nodes ensures that each identifier only exists once. The disadvantage of this approach is that the data will not be stored in the .blend file. For eg caching that is no problem though. 

This is a 'problem' that is caused by Blender. Animation Nodes (or other addons) cannot change this behavior. Personally I don't care that I can't use the instance dictionary. Maybe it is even better the way it is because so we cannot confuse the data that is stored in the .blend file and other variables.